Which of these compounds is considered a sulfur compound?

Cinnabar is considered a sulfur compound because it is primarily composed of mercury sulfide (HgS). This mineral often serves as a significant source of mercury and is characterized by its bright red color. The presence of sulfur in its chemical structure categorizes it within the group of sulfur compounds.

In contrast, alumina (a form of aluminum oxide), ammonium nitrate (a nitrogen-containing compound), and calcium carbonate (composed of calcium, carbon, and oxygen) do not contain sulfur in their chemical formulas or structures, which disqualifies them from being classified as sulfur compounds. Thus, cinnabar stands out as the only option that inherently incorporates sulfur in its composition.